Field of the Invention
The present invention relates to a manufacturing method and apparatus for manufacturing a silicon carbide epitaxial wafer.
Background Art
In recent years, a silicon carbide (hereinafter referred to as SiC) semiconductor has attracted attention mainly as a material for power devices for power control because its bandgap, dielectric breakdown electric field intensity, saturated drift speed and thermal conductivity are higher than those of silicon semiconductors. In fact, a power device using the SiC semiconductor is capable of largely reducing power loss, being reduced in size and realizing energy saving at the time of power supply power conversion and can therefore be a key device for, for example, improving the performance of electric vehicles and improving the functions of solar battery systems or the like in realization of a low-carbon society.
In making a SiC power device, a layer to be provided as an active region of the semiconductor device is generally grown epitaxially on a SiC bulk monocrystal substrate by thermal chemical vapor deposition (thermal CVD) or the like. The term “active region” refers here to a region which is built by precisely controlling doping concentration in a crystal and its film thickness, and which has a section containing a growth direction axis. The reason why such an epitaxially grown layer is required as well as a bulk monocrystal substrate is that a doping concentration and a film thickness are generally prescribed in accordance with the specifications of a device and there is a demand for obtaining the doping concentration and the film thickness with accuracies higher than those attained for bulk monocrystal substrates in ordinary cases.
A wafer having an epitaxially grown layer formed on a SiC bulk monocrystal substrate will hereinafter be referred to as “epitaxial wafer”. A silicon carbide semiconductor device is fabricated by performing various kinds of processing on a silicon carbide epitaxial wafer. If the silicon carbide epitaxial wafer has defects, the silicon carbide semiconductor device locally fails to hold a high voltage, resulting in generation of a leak current. If the density of such defects is increased, the nondefective rate in manufacturing of the silicon carbide semiconductor device is reduced.
When the silicon carbide epitaxial wafer is manufactured, silicon carbide is attached not only to the wafer but also to inner walls of a growth furnace and to surfaces of a wafer holder on which the wafer is mounted. The attached silicon carbide is structurally brittle and can therefore form silicon carbide particles easily. If silicon carbide particles are attached to the wafer surface, crystal defects such as downfalls and triangular defects are generated starting from the attachment points.
As a method for reducing silicon carbide particles which can be a cause of such crystal defects, a method of cleaning the wafer holder by etching silicon carbide particles attached to the wafer holder with chlorine trifluoride has been proposed (see, for example, Japanese Patent No. 5542560). A method of removing silicon carbide particles by using a cleaning gas containing iodine heptafluoride without etching and damaging graphite which is the basic material of a certain member has also been proposed (see, for example, Japanese Patent Laid-Open No. 2014-154865).
In the case of the method disclosed in Japanese Patent No. 5542560, not only the attached silicon carbide but also the silicon carbide film formed as protective film on internal members of the growth furnace and the wafer holder is etched and it is, therefore, difficult to perform cleaning management. Also, special expelling equipment is required to enable use of chlorine trifluoride, for example, because it corrodes the apparatus and piping. The method thus entails problems in terms of operation and cost. Further, because the rate of etching of silicon carbide is lower than that of etching of silicon, a long time is taken to complete cleaning and it is difficult to completely remove silicon carbide.
In the case of the method disclosed in Japanese Patent Laid-Open No. 2014-154865, iodine heptafluoride, which does not etch graphite, is used but the silicon carbide film formed as protective film on the wafer holder and internal members of the growth furnace are thereby etched, as in the case of the method disclosed in Japanese Patent No. 5542560, and it is difficult to perform cleaning management. Thus, use of a fluorine-based gas as cleaning gas entails the problem of members other than the attached silicon carbide being etched.